How to make a large mesh smaller and workable?
Some ways to reduce interaction time. Consider Please


Work in wireframe mode when that is possible.  You need not work in render mode all the time.
Change maximum draw type for some objects to wireframe
Turn off Eye display of less important items for preview in outliner.  Camera Render Stays On.  Sometimes Groups can help this.
Bill board texture technique game style. A complex fence might be rendered once and then the texture put on blender planes. Crude and quick display substitute. Eye Display Only. Camera Render off.  Same location as the complex fence.
Working in local mode sometimes to isolate editing ... key / to enter and the same key to exit


Once a array modifier with repeat N on a meshA is applied and become meshFinal ... 


the number of vertices of MeshFinal = N * (number of vertices of MeshA)


Once can reduce such vertices sometimes by using a Particle System with can produces meshes with a shared vertex datablock.   Configuration Settings .. Editing ... Duplicate Data ... Mesh may affect this.  The object size overhead is present in this case.